Electron beam apparatuses, in particular a scanning electron microscope (also referred to as SEM below) and/or a transmission electron microscope (also referred to as TEM below), are used to examine objects (also referred to as samples) in order to obtain knowledge in respect of the properties and behavior of the objects under certain conditions.
In an SEM, an electron beam (also referred to as primary electron beam below) is generated by means of a beam generator and focused on an object to be examined by way of a beam guiding system. An objective lens is used for focusing purposes. The primary electron beam is guided in a raster manner over a surface of the object to be examined by way of a deflection device. Here, the electrons of the primary electron beam interact with the object to be examined. Interaction particles in particular are produced as a result of the interaction. By way of example, the interaction particles are electrons. In particular, electrons are emitted by the object—the so-called secondary electrons—and electrons of the primary electron beam are scattered back—the so-called backscattered electrons. The interaction particles form the so-called secondary beam and are detected by at least one particle detector. The particle detector generates detection signals which are used for generating an image of the object. An image of the object to be examined is thus obtained.
In the case of a TEM, a primary electron beam is likewise generated by means of a beam generator and focused on an object to be examined by means of a beam guiding system. The primary electron beam passes through the object to be examined. When the primary electron beam passes through the object to be examined, the electrons of the primary electron beam interact with the material of the object to be examined. The electrons passing through the object to be examined are imaged onto a luminescent screen or onto a detector—for example in the form of a camera—by a system comprising an objective. By way of example, the aforementioned system additionally also comprises a projection lens. Here, imaging can also take place in the scanning mode of a TEM. Such a TEM is generally referred to as STEM. Additionally, provision can be made for detecting electrons scattered back at the object to be examined and/or secondary electrons emitted by the object to be examined by means of a further detector in order to image an object to be examined.
The integration of the function of an STEM and an SEM in a single particle beam apparatus is known. It is therefore possible to carry out examinations of objects with an SEM function and/or with an STEM function using this particle beam apparatus.
Furthermore, the prior art teaches the practice of analyzing and/or processing an object in a particle beam apparatus using, on the one hand, electrons and, on the other hand, ions. By way of example, an electron beam column having the function of an SEM is arranged at the particle beam apparatus. Additionally, an ion beam column is arranged at the particle beam apparatus. Ions used for processing an object are generated by means of an ion beam generator arranged in the ion beam column. By way of example, material of the object is ablated or material is applied onto the object during the processing. The ions are additionally or alternatively used for imaging. The electron beam column with the SEM function serves, in particular, for examining further the processed or unprocessed object, but also for processing the object.
In a further known particle beam apparatus, from the abovementioned secondary beam a large portion of the secondary particles is separated from the backscattered particles. By way of example, a large portion of the secondary electrons is blocked out from the secondary beam. Almost only backscattered electrons then reach the detector. Said backscattered electrons are detected by the detector. Detection signals are generated which are used for imaging purposes. The known particle beam apparatus has an opposing field grid arranged between the object and the detector. A voltage is applied to the opposing field grid in such a way that a large portion of the secondary electrons is reflected by the opposing field grid and not detected by the detector.
The abovementioned opposing field grid is also used to determine the energy of the interaction particles. As already mentioned above, a voltage is applied to the opposing field grid in such a way that a large portion of the secondary particles, for example of the secondary electrons, is reflected by the opposing field grid and not detected by the detector. The voltage determines a threshold energy. Interaction particles having an energy that is greater than the threshold energy predefined by the opposing field grid reach the detector and are detected. It is thus possible to make a statement to the effect that the detected interaction particles have an energy that is greater than the threshold energy.
In a further known particle beam apparatus, provision is made for arranging, in addition to the opposing field grid mentioned above, an electrostatic aperture between the opposing field grid and the detector. By applying a voltage to the electrostatic aperture, an electric field is generated in such a way that only interaction particles which have the threshold energy or substantially have the threshold energy impinge on the detector.
It is furthermore known to design the opposing field grid in a curved fashion. A spherical opposing field is generated by means of this opposing field grid and makes it possible that interaction particles which pass both near to and far from an optical axis of the particle beam apparatus and which pass from a point on the optical axis and form a divergent beam pass through the opposing field grid and reach the detector if the interaction particles have at least the threshold energy.
In yet another known particle beam apparatus, provision is made for arranging, as viewed in a direction opposite to the direction of incidence of the interaction particles on the detector, firstly an opposing field grid and then a magnetic or electrostatic lens. This makes it possible to cause a divergent bundle of the beam of interaction particles to enter the opposing field of the opposing field grid in a parallel fashion.
With regard to the prior art, reference is made to EP 1 439 565 B1, US 2014/0284476 A1, DE 199 29 185 A1, WO 2008/087384 A2 and WO 2008/087386 A1.
The bundle of the beam of interaction particles, for example the secondary beam mentioned above, may be very large. By way of example, the bundle of the secondary beam may have a divergence angle of approximately ±40 mrad. This means that some of the interaction particles of the secondary beam pass near to the optical axis of the particle beam apparatus. Still other interaction particles of the secondary beam pass far from the optical axis. In the particle beam apparatus having both the abovementioned magnetic or electrostatic lens and the abovementioned opposing field grid, the interaction particles that pass near to the optical axis pass parallel to the optical axis to a good approximation after passing through the magnetic or electrostatic lens. However, interaction particles that pass far from the optical axis, after passing through the magnetic or electrostatic lens, do not pass parallel to the optical axis, but rather in the direction of the optical axis, on account of an aperture aberration of the magnetic or electrostatic lens. This may lead to errors in the detection of the interaction particles. It is possible that not all the interaction particles which actually have the threshold energy reach the detector.
The abovementioned problem may be solved if the opposing field grid arranged between the detector and the magnetic or electrostatic lens is curved in such a way that the starting point of the radius of curvature corresponds to a crossover of the secondary beam on the optical axis. An explanation is given below of what is understood by the terms starting point and crossover. The starting point is a point on the optical axis passing through the opposing field grid. An intersection point of the optical axis with the opposing field grid is a center of curvature of the curvature. The radius of curvature of the curvature corresponds to the section between the center of curvature and the starting point. A crossover is understood to be a position on an axis, for example the optical axis of a particle beam apparatus, at which particles, for example the electrons of the primary electron beam or particles of the secondary beam, converge and a cross-sectional area of the beam, for example of the primary electron beam or of the secondary beam, has a local minimum.
It may then happen that the crossover of the secondary beam of interaction particles travels along the optical axis on account of changed imaging properties of the particle beam apparatus. Since the curvature of the known opposing field grid cannot be changed or can be changed only with very great difficulty, an adaptation to the altered position of the crossover is not possible or is possible only with very great difficulty, such that the interaction particles passing on paths far from the optical axis possibly do not reach the detector.
Accordingly, it is desirable to be able to specify a simplified analysis device for analyzing the energy of charged particles and a particle beam apparatus comprising such an analysis device in which a displacement of a crossover of the secondary beam of interaction particles along an optical axis can be compensated for.